As a promising clean fuel for vehicles, hydrogen can be used for propulsion, either directly or in fuel cells. One limitation to using hydrogen as a propulsion fuel is discovering a suitable high capacity hydrogen-storage material that will make this source of energy economically viable. Although there have been numerous materials systems studied as potential candidates for hydrogen storage applications, none of the materials known to date have demonstrated sufficient hydrogen capacity or efficiency at commercially viable temperature ranges.
Following work on Ti-doped NaAlH4, alkali- and alkaline-earth-metal-based complex aluminum hydrides, such as MAlH4 (M=Li, Na, K), have been studied intensively as possible candidate materials for high-capacity reversible hydrogen storage. It has been generally found that alkali-based aluminum hydrides have great potential for storing hydrogen at moderate temperatures and pressures. Typically, the dehydrogenation of these complex hydrides follows the following 2-step reactions:
                                          M            ⁢            AlH                    4                →                                            1              3                        ⁢                          M              3                        ⁢            Al            ⁢                                                  ⁢                          H              6                                +                                    2              3                        ⁢            Al                    +                      H            2                                              (        1        )                                                      1            3                    ⁢                      M            3                    ⁢          A          ⁢                                          ⁢          l          ⁢                                          ⁢                      H            6                          →                              M            ⁢            H                    +                                    1              3                        ⁢            Al                    +                                    1              2                        ⁢                          H              2                                                          (        2        )            Among them, lithium alanate (LiAlH4), which was first synthesized in 1947 and is used as a powerful reducing agent in organic chemistry, is one of the materials having one of the highest inherent hydrogen storage capacities with a total hydrogen content of approximately 10.6 wt %.
During decomposition at elevated temperatures, LiAlH4 first reduces to various intermediate compounds, including Li3AlH6, and then to LiH at about 160 to about 210° C., liberating 5.3 wt % and 2.6 wt % of hydrogen, respectively. The dehydrogenation of LiH, however, occurs at much higher temperatures, around 720° C., liberating another 2.6 wt % of hydrogen. This temperature is too high for commercial use, and hence the decomposition of LiH is not considered a viable dehydrogenation reaction for practical purposes. Without the decomposition of LiH, the maximum potential hydrogen storage capacity of LiAlH4 decreases to 7.9 wt % for LiAlH4. The difficulty of releasing H2 from LiH limits the practical hydrogen capacity of LiAlH4. A similar problem also exists with using the NaAlH4 system as a hydrogen storage composition.
On the other hand, when lithium hydride is reacted with lithium amide (LiNH2), hydrogenation and dehydrogenation are accomplished by the following two-step reversible reactions:LiNH2+LiHLi2NH+H2  (3)Li2NH+LiHLi3N+H2  (4)The dehydrogenation temperature is from about 200 to about 430° C., and the dehydrogenation reactions can release all of the available hydrogen from LiNH2 and LiH. Reactions (3) and (4) indicate that the dehydrogenation temperature of LiH can be decreased dramatically when it is combined with LiNH2. However, the reaction temperature of Reaction (4) is still too high for practical applications.
Considering all Reactions (1)-(4) collectively, the hydrogen in LiAlH4 can be released by combining LiAlH4 with LiNH2. The overall reaction for the combined LiAlH4 and LiNH2 system can be given as:LiAlH4+LiNH2→Al+Li2NH+2.5H2  (5)Thermogravimetric analysis (TGA) of LiALH4/LiNH2 mixtures without any catalysts indicated that a large amount of hydrogen (˜8.1 wt %) is released from about 80° C. to about 320° C. under a heating rate of 2° C./min in three dehydrogenation reaction steps. The results also showed that the dehydrogenation temperature of the LiAlH4/LiNH2 system is lower than that of either Ti-doped LiAlH4 or LiNH2/LiH. The percent of hydrogen released from a LiAlH4/LiNH2 mixture is also higher than that from LiAlH4 or LiNH2/LiH. In effect, the compound LiNH2 destabilizes LiAlH4 by reacting with LiH during the dehydrogenation process of LiAlH4.
However, in addition to the high inherent hydrogen storage capacity and good dehydrogenation kinetics, hydrogen release and uptake reactions should be reversible for a hydrogen storage material to be economically viable. Reaction (5) can be only partially reversed to the step of forming Li3AlH6. The reverse reaction from Li3AlH6 to LiAlH4 seemed, however, not feasible. Therefore, reversible hydrogen storage compositions continue to be sought through ongoing research and development.